The present invention relates to a protective circuit and a protective element incorporated therein for protecting a rechargeable battery such as a lithium ion rechargeable battery against overcharging.
Charging of rechargeable batteries beyond their suitable charge conditions leads to the generation of gas or heat due to decomposition of electrolyte and may cause breakdown or deterioration of the battery. Lithium-based rechargeable batteries are particularly susceptible to deterioration or damage when operated at a voltage exceeding a predetermined range. It is therefore the normal practice to provide a battery protection device for protecting the battery from improper use.
The battery protection devices include those which are mounted in the rechargeable battery itself as a PTC (Positive Temperature Coefficient) element or a current shutoff valve, and those configured as a circuit substrate on which is formed a battery protective circuit that shuts off the charging and discharging circuit of the rechargeable battery in response to an abnormal state, for instance. The above-mentioned PTC device is serially connected to the charging and discharging circuit of the rechargeable battery, and generates heat itself when subjected to an excessively large current, thereby preventing the further flow of the excessively large current through a rapid increase in resistance caused by the rise in temperature. With a relatively large rechargeable battery, this PTC device is provided inside the sealing assembly of the battery. The above-mentioned current shutoff valve is normally installed in the sealing assembly, and when gas is generated within the rechargeable battery, it is deformed by the rise in internal pressure and breaks when the internal pressure exceeds a permissible value, thereby releasing the gas and, through its breakage, shutting off the flow of current to the charging and discharging circuit. PCT elements and current shutoff valves are well known as mechanisms provided in relatively large, cylindrical, lithium ion rechargeable batteries.
The above-mentioned battery protective circuit is disclosed, for example, in Japanese Patent No. 2,872,365, and is configured as shown in FIG. 12. The voltage of a rechargeable battery 30 is detected by a control means 33, and when a voltage over a predetermined charge-permitting voltage is detected, a MOSFET 31 serially connected to a charging and discharging circuit is put in an OFF state, whereby the charging and discharging circuit is shut off, and charging current is impeded. When a voltage below a predetermined discharge-permitting voltage is detected, a MOSFET 32 serially connected to the charging and discharging circuit is put in an OFF state, whereby the charging and discharging circuit is shut off, and discharging current is impeded. This control allows the rechargeable battery 30 to be protected against damage or diminished performance due to overcharging or overdischarging.
However, when the battery protective circuit is not operating properly, and particularly when the anti-overcharging function is not operating, there is the danger that the electrolyte will decompose as the overcharging state progresses and that the rechargeable battery 30 will be ruptured by the gas generated as the temperature rises. In view of this, as shown in FIG. 13, a battery protective circuit configuration has been proposed in which there are provided a control means 34 for preventing overcharging, overdischarging, and over-current, and an overcharging control means 36 for halting the overcharging state if this control means 34 should malfunction.
With this configuration, the control means 34 prevents overcharging, overdischarging, and over-current, and when an operational aberration occurs, such as a breakdown or malfunction in the ability of this control means 34 to prevent overcharging, a voltage corresponding to an overcharging state is detected by the overcharging control means 36, and the overcharging control means 36 puts the MOSFET 35 in an ON state. When this MOSFET 35 is turned ON, a resistor 18 generates heat which melts a heat-coupled temperature fuse 19, shutting off the charging and discharging circuit of the rechargeable battery 30.
It is thereby possible to avoid a prolonged overcharging state, which is the most detrimental state for the rechargeable battery 30 to be in. Since the structure for preventing overcharging is provided redundantly, and the generation of gas due to a prolonged overcharging state is prevented, it is also possible to eliminate the above-mentioned current shutoff valve that mechanically shuts off the power circuit.
Nevertheless, although an anti-overcharging circuit that is redundantly provided as in the above conventional structure was indeed effective at preventing battery breakdown due to a prolonged overcharging state, because the second control means, which was actuated when something was amiss with the first control means, actuated a non-resettable shutoff means, the operation thereof could not be tested, so it was impossible to ensure proper operation and obtain high reliability by testing individual operating states. The proper operation of an overcharging protective circuit is essential with rechargeable batteries of high energy density, such as a lithium ion rechargeable battery, and it is necessary to be able to ensure the reliability of individual batteries or battery packs by testing the operation of the protective circuit.
Meanwhile, a temperature fuse, heating means, and so forth are parts that cannot be incorporated into an integrated circuit, and therefore require their own installation space, and this hinders designing a compact battery pack using small rechargeable batteries, or designing a rechargeable battery with an attached protective circuit in which the protective circuit is integrated with the rechargeable battery.
Making rechargeable batteries smaller is very important in terms of making portable devices more compact, and even with this smaller size, the energy density per unit of volume still needs to be increased. An effective means for achieving this is to use an electrical rather than mechanical current shutoff valve to shut off the power circuit, and we are awaiting the development of a protective circuit that effectively protects a rechargeable battery from a prolonged overcharging state, with a smaller structure for the electrical shutoff of the power circuit than that in prior art, as well as a protective element used in this protective circuit.
It is an object of the present invention to provide a protective circuit with a compact structure that affords reliable protection of a rechargeable battery, and at the same time, a protective element that is compatible with this protective circuit.
To achieve the stated object, the present invention provides a battery protective circuit incorporated in a rechargeable battery, comprising:
a first switching means and a second switching means connected in series in a charge/discharge circuit of the rechargeable battery;
a main protective circuit for
detecting a battery voltage between positive and negative electrodes of the rechargeable battery;
turning on the first switching means when the battery voltage is below a charge-prohibiting voltage, above which charging of the rechargeable battery must be prohibited; and
turning off the first switching means when the battery voltage is detected to be higher than said charger-prohibiting voltage, and maintaining the OFF state of the first switching means until the detected voltage becomes below a charge-permitting voltage that is lower than said charge-prohibiting voltage; and
a sub-protective circuit for
detecting the battery voltage between the positive and negative electrodes of the rechargeable battery;
turning on the second switching means when the battery voltage is below a second charge-prohibiting voltage which is higher than the first charge-prohibiting voltage; and
turning off the second switching means when the battery voltage is detected to be higher than the second charge-prohibiting voltage.
With the above structure, even in the event of a breakdown or malfunction of the main protective circuit, the sub-protective circuit detects the overcharging and shuts off the charging and discharging circuit of the rechargeable battery. This affords a redundant overcharging protective circuit, which more effectively prevents the overcharging of the rechargeable battery and keeps the overcharging from progressing to the point of the breakdown of the rechargeable battery. This redundant overcharging protection allows the operation of overcharging protective circuits to be tested for each circuit, and makes possible more reliable battery protection. Also, since a redundant overcharging protective circuit allows overcharging to be effectively prevented, there is none of the gas generation that would accompany overcharging, making it possible to eliminate mechanisms such as a gas escape valve for releasing abnormal internal pressure in a battery caused by gas generation. Therefore, no space is needed for providing such gas escape valve or the like, making it easier to design a compact rechargeable battery. Also, since the circuit can be made up of semiconductor elements, the protective circuit can consist of an integrated circuit, making it possible to reduce the size of a battery pack and fit the protective circuit inside a rechargeable battery.
In the above structure, the sub-protective circuit turns off the second switching means through detection of the second charge-prohibiting voltage, and maintains this OFF state until a second charge-permitting voltage is detected. Specifically, the second charge-permitting voltage is set below the first charge-prohibiting voltage, the result of which is that the second switching means is turned OFF by the sub-protective circuit, rendering charging impossible. Also, the shutoff operation of the second switching means prevents the circuit from returning from an OFF state to an ON state, and the state of overcharging protection from being ended, due to a decrease in battery voltage when the charging circuit is opened.
Alternatively, the sub-protective circuit may fix the OFF state of the second switching means after the second charge-prohibiting voltage has been detected. By fixing the OFF state of the second switching means when overcharging is detected, the rechargeable battery can be reliably protected against loss of battery protection function caused by malfunction of the main protective circuit.
The first and second switching means may be constructed of power MOSFETs having parasitic diodes therein, and connected such that the forward direction of the parasitic diodes is the discharge direction of the rechargeable battery. Thereby, even if the first and second switching means are in a shutoff state due to the detection of an overcharging state, discharge will be possible through the parasitic diodes, and the rechargeable battery can be used even in a state in which the anti-overcharging function has been actuated.
To achieve the stated object, the present invention also provides a battery protective circuit incorporated in a rechargeable battery, comprising:
a first switching means and a second switching means connected in series in a charge/discharge circuit of the rechargeable battery;
a main protective circuit for controlling the first switching means in accordance with a battery voltage across positive and negative electrodes of the rechargeable battery and a discharge current of the rechargeable battery; and
a sub-protective circuit for controlling the second switching means in accordance with the battery voltage across positive and negative electrodes of the rechargeable battery, wherein:
the first switching means and the second switching means are turned on, when the rechargeable battery is in a normal condition wherein the voltage across the positive and negative electrodes of the rechargeable battery is within a range above a discharge-prohibiting voltage and below a first charge-prohibiting voltage, said discharge-prohibiting voltage being a limit value below which discharging of the rechargeable battery must be prohibited, and said charge-prohibiting voltage being a limit value above which charging of the rechargeable battery must be prohibited;
the first switching means is turned OFF when the detected discharge current is above a predetermined value;
the first switching means is put in a charging direction OFF/discharging direction ON state when the detected voltage is above said first charge-prohibiting voltage, and maintained in said charging direction OFF/discharging direction ON state until a first charge-permitting voltage, that is lower than the first charge-prohibiting voltage, is detected;
the first switching means is put in a discharging direction OFF/charging direction ON state when the detected voltage is below said discharge-prohibiting voltage, and maintained in said discharging direction OFF/charging direction ON state until a discharge-permitting voltage, that is higher than said discharge-prohibiting voltage is detected;
the second switching means is turned off when the detected voltage is above a second charge-prohibiting voltage, which is higher than said first charge-prohibiting voltage, and maintained in the OFF state until a second charge-permitting voltage, that is lower than said second charge-prohibiting voltage, is detected.
With the above structure, the main protective circuit detects the voltage and discharge current of the rechargeable battery, and during normal operation puts the rechargeable battery in a usable state by turning ON switching elements, but in response to an abnormal state, this main protective circuit either turns OFF the first switching means or renders it capable of only charging or discharging. If something should go amiss in this main protective circuit, such as a breakdown or malfunction, and it should stop preventing overcharging, then the sub-protective circuit detects the overcharging and shuts off the charging and discharging circuit of the rechargeable battery so there is a redundant overcharging protective circuit, with which the overcharging of the rechargeable battery is effectively prevented, and overcharging is kept from progressing to the point of the breakdown of the rechargeable battery. This redundant overcharging protection allows for the individual testing of the operation of these circuits, and makes possible more reliable battery protection. Also, since a redundant overcharging protective circuit allows overcharging to be effectively prevented, there is none of the gas generation that would accompany overcharging, eliminating the need for space to provide a gas escape valve or the like and facilitating the design of a compact rechargeable battery. Also, the circuit can be made up of semiconductor elements, so the protective circuit can consist of an integrated circuit, making it possible to reduce the size of a battery pack and fit the protective circuit inside a rechargeable battery.
In the above structure, the first switching means is an FET with no parasitic diode in its interior, and the various ON/OFF states are assumed according to the gate voltage thereof. Thus a single FET can prevent overcharging, overdischarging, and over-current, which allows the battery protective circuit to be more compact.
To achieve the stated object, the present invention also provides a battery protective circuit incorporated in a rechargeable battery, comprising:
a PTC device connected in series in a charge/discharge circuit of the rechargeable battery;
heating means heat-coupled to the PTC element;
a first switching means connected to the PTC device for controlling power supply to said heating means;
a second switching means connected in series in the charge/discharge circuit of the rechargeable battery;
a main protective circuit for
detecting a battery voltage between positive and negative electrodes of the rechargeable battery;
turning off the first switching means when the detected voltage is below a first charge-prohibiting voltage, above which charging of the rechargeable battery must be prohibited; and
turning on the first switching means for supplying power to the heating means when the detected voltage is above the first charge-prohibiting voltage, and maintaining said ON state of the first switching means until a first charge-permitting voltage, that is lower than said first charge-prohibiting voltage, is detected; and
a sub-protective circuit for
detecting the battery voltage between the positive and negative electrodes of the rechargeable battery;
turning on the second switching means when the detected voltage is below a second charge-prohibiting voltage that is higher than the first charge-prohibiting voltage; and
turning off the second switching means when the detected voltage is above the second charge-prohibiting voltage, and maintaining the OFF state of the second switching means until a second charge-permitting voltage, which is lower than said second charge-prohibiting voltage, is detected.
With the above structure, when an overcharging state is detected from the voltage of the rechargeable battery, the main protective circuit turns on the first switching means and sends power to the heating means, and the PTC device serially connected to the charging and discharging circuit of the rechargeable battery is heated by this heating means. The resistance of the PTC device increases rapidly as the temperature rises, which restricts charging current to the rechargeable battery and protects the battery from overcharging. If this main protective circuit should stop preventing overcharging due to a problem such as a breakdown or malfunction, then the sub-protective circuit detects the overcharging and shuts off the charging and discharging circuit of the rechargeable battery so there is a redundant overcharging protective circuit, with which the overcharging of the rechargeable battery is effectively prevented, and overcharging is kept from progressing to the point of the breakdown of the rechargeable battery. This redundant overcharging protection allows for the individual testing of the operation of these circuits, and makes possible more reliable battery protection. Also, since a redundant overcharging protective circuit allows overcharging to be effectively prevented, there is none of the gas generation that would accompany overcharging, making it possible to eliminate mechanisms such as a gas escape valve for releasing abnormal internal pressure in a battery caused by gas generation. Therefore, no space is needed for providing this gas escape valve or the like, making it easier to design a compact rechargeable battery. Also, the circuit can be made up of semiconductor elements, so the protective circuit can consist of an integrated circuit, making it possible to reduce the size of a battery pack and fit the protective circuit inside a rechargeable battery.
In the above structure, the heating means can be constructed of a second PTC device heat-coupled to the first PTC device. Both PTC devices may be formed flat, so that a good heat-coupling state can be obtained when two PTC devices are joined together on their flat sides. Thus the charge current-restricting construction can be made compact, since it is only necessary to send power to the second PTC device, thereby heating the first PTC device and increasing its resistance.
To achieve the stated object, the present invention also provides a battery protective circuit for a rechargeable battery, comprising:
voltage detection means connected in series between positive and negative electrodes of the rechargeable battery for detecting a battery voltage and outputting a control signal when a voltage exceeding a predetermined value is detected;
a PTC device serially connected to the voltage detection means;
heating means, which heats up by electrical conduction, connected to the voltage detection means and heat-coupled to the PTC device; and
switching means for turning on the heating means in accordance with the control signal from the voltage detection means.
Power is sent to the heating means by the actuation of the switching means through a control signal outputted when the voltage detection means detects a voltage exceeding a specific value, such as the voltage resulting from prolonged overcharging, and the heating means heats the heat-coupled PTC device. A PTC device is characterized by exhibiting a positive coefficient resistance change with respect to temperature, and in particular by entering a tripped state in which resistance increases rapidly above a specific critical temperature. Normally, the resistance is very low and the drop in voltage caused by input and output current of the rechargeable battery is so small that it does not interfere with the input and output circuit, but when an excessively large current flows in, self-generation of heat results in a sharp increase in resistance, which prevents excessive current flow. The resistance of this PTC device rises sharply by the elevated temperature when heated by the heating means, and this restricts the current of the input and output circuit of the rechargeable battery. Therefore, when the voltage detection means detects a voltage indicating an abnormal state such as a prolonged overcharging state, the tripping of the PTC device restricts the input and output current of the rechargeable battery so damage to the rechargeable battery due to prolonged overcharging is prevented.
In the above structure, the heating means can consist of a second PTC device heat-coupled to the above-mentioned PTC device. When power is sent to the second PTC device and the temperature raised, the heat-coupled PTC device is heated and the PTC device tripped.
The present invention also provides a battery protective circuit for a rechargeable battery, comprising:
voltage detection means connected in series between positive and negative electrodes of a rechargeable battery for detecting a battery voltage and outputting a control signal when a voltage exceeding a predetermined value is detected;
a temperature fuse serially connected to the rechargeable battery;
a heating PTC device heat-coupled to the temperature fuse; and
switching means for turning on the heating PTC device in accordance with the control signal from the voltage detection means.
Power is sent to the heating PTC device by the actuation of the switching means through a control signal outputted when the voltage detection means detects a voltage exceeding a specific value, such as the voltage resulting from prolonged overcharging, and the heating PTC device raises the temperature through current flow, which melts the heat-coupled temperature fuse. Therefore, the input and output circuit of the rechargeable battery is shut off when the voltage detection means detects a voltage indicating an abnormal state such as a prolonged overcharging state, so damage to the rechargeable battery due to prolonged overcharging is prevented.
In the above structures, the rechargeable battery is redundantly protected against overcharging damage by setting the voltage exceeding a specific value detected by the voltage detection means to be higher than the voltage at which an overcharging state is detected. Specifically, when a control circuit that detects a state such as overcharging or overdischarging and shuts off the input and output circuit of the rechargeable battery is separately configured, the initial value of the overcharging state is detected by this control circuit and overcharging is prevented, but if something goes wrong with this control circuit, rupture or other such damage to the rechargeable battery can be caused by prolonged overcharging. If the structure is such that the voltage detection means outputs a control signal through the detection of a voltage higher than the above-mentioned initial value, a voltage raised by prolonged overcharging will be detected, action will be taken to prevent overcharging, and the rechargeable battery will be protected even if something goes wrong with the control circuit.
The present invention also provides a protective element incorporated in a battery protective circuit for a rechargeable battery, comprising a plurality of PTC devices formed in a flat shape and laminated in a heat-coupled state. One of the PTC devices is serially connected to the rechargeable battery, and another PTC device is connected to a power control circuit controlling the power conduction thereof. When power is sent to the second PTC device heat-coupled to the PTC device on the side serially connected to the rechargeable battery, the heating caused by current flow raises the temperature of the PTC device serially connected to the rechargeable battery, which sharply increases the resistance and trips the PTC device, at which point the input and output current of the rechargeable battery is restricted. Thus the plurality of PTC devices effectively act as a protective element for the rechargeable battery, by constructing the power control circuit such that power is sent to the PTC device on the heated side upon detection of an abnormal state in the rechargeable battery such as overcharging.
In the above structure, the PTC devices can comprise a combination of shapes, sizes, and electrical characteristics selected as desired, and a suitable combination can be selected according to the configuration of the protective circuit.
To be more specific, the PTC device may be constituted with two PTC elements formed in a flat shape and joined together via an electrode material interposed between the flat sides thereof, each of the PTC elements being respectively provided with electrode materials joined to their outer sides, so that the PTC device will take up less space.
The electrode materials may be formed from a copper-nickel alloy or a clad material made from a copper-nickel alloy and nickel, so that the product will lend itself very well to soldering and welding, and will also have excellent electrical and thermal conductivity. The use of these materials facilitates work and enhances electrical and mechanical performance.
Leads can also be formed extending from the electrode materials, which facilitates circuit connections.
Leads can also be formed extending from the electrode materials in two mutually opposite directions, or in various different directions, the selection of which can be made according to the circuit configuration.
The electrode materials may be formed in smaller outer dimensions than the PTC elements to which they are joined, so that it will be easier to obtain a good solder joint between the electrode materials and the PTC devices.
The present invention also provides a protective element incorporated in a battery protective circuit for a rechargeable battery comprising a temperature fuse and a PTC device heat-coupled to the temperature fuse. When the temperature fuse is serially connected to the rechargeable battery, and the PTC device is connected to the power control circuit that controls the power conduction thereof, the heating caused by current flow melts the temperature fuse and shuts off the input and output current of the rechargeable battery. Thus the temperature fuse and PTC device will effectively act as a protective element for the rechargeable battery, by constructing the power control circuit such that power is sent to the PTC device upon detection of an abnormal state in the rechargeable battery such as overcharging.
The plurality of heat-coupled PTC devices may be covered with a thermally insulating material, so that the diffusion of heat will be suppressed and heat coupling will be more effective.